The present invention relates to a process of self-alignment of differently doped regions, particularly the regions of the emitters and of the base contacts, in particular for the manufacture of transistors and integrated circuits.
In the field of production of semiconductor devices having high performances, concerning for example the switching time, the frequency gain, the signal/noise ratio, etc., all the parasite elements must be reduced, which involves in general a greater and greater miniaturization of all or part of the dimensions. Independently of the performances, this miniaturization is desirable for increasing the integration density in the case of integrated circuits. Insofar as the depth is concerned, reduction in this direction is conditioned by the mastery of the distribution profiles of the doping impurities, which is acquired for example by means of ionic implantation. But, insofar as the surface extent is concerned, an important limiting factor is the accuracy which may be expected in the relative positioning of the apertures provided in the basic structure and serving to define the different regions to be doped, i.e. the regions of the emitters and the base contacts.
In the prior art, it is known to use for this purpose a so-called "self-aligning" technique, in which relative positioning is achieved of two apertures and of regions with different doping obtained from these apertures and this so that the apertures or the regions do not overlap, while being situated at a relative distance as reduced as possible. There will be described hereafter, with reference to FIGS. 1 to 11 accompanying the present application, two known examples of implementing this technique in the case of the manufacture of an ultra-high frequency transistor, of the type formed with a conventional interdigitated structure, in which the emitter of the transistor is formed by fingers alternating with base contacts.
In a conventional technique (FIG. 1), there is deposited on a substrate 1, for example of type n.sup.+, an epitaxial layer of the same type (n) 2. After depositing a masking layer, for example an oxide layer 3, apertures are provided by photo-etching, through which p-type impurities are introduced so as to create weakly resistive zones 4, which are intended to receive subsequently a contact metal. Then another aperture 5 is formed in the masking oxide layer and base 6 is formed by ionic implantation or by diffusion. Then a new, for example oxide, masking layer 7 is deposited on base 6. Then the emitter regions 8 are formed by the known technique of photo-etching, the emitters then being obtained by diffusion or implantation of impurities, of type n in the case considered. The end of the procedure, relative to the photo-etching of the contact zones, to the metal deposit and to the photo-etching of the interconnections, is then carried out in a known way and is therefore not shown in the figure.
With the procedure outlined above, the distance which may be obtained between the emitters cannot be much less than 3 microns without using the self-aligning technique, which enabled this figure to be lowered in a reproducible way to 1 micron or to a few tenths of a micron, so the factor of merit of the device to be improved, which depends among other things intimately on this distance.
According to a first process known under the name of composite masking, this distance may be reduced to 2 microns in a reproducible way, by inscribing all the apertures, corresponding to the different regions to be doped, in a "memory" mask and by successively selecting each of the apertures by means of a selection mask. The sequence of the operations is reproduced in the accompanying FIGS. 2 to 5. On a substrate 9 of type n.sup.+ for example there is deposited an epitaxial layer 10 of the same type and, after formation of a masking layer, for example an oxide layer 11, an aperture is formed whose edges are shown at 12 and at the level of which base 13 is formed by diffusion or implantation of ions. Then a masking layer 14, for example an oxide layer, is deposited to give the structure of FIG. 2. Then, by means of a composite mask 15 (FIG. 3) having apertures 16 for forming the emitters and apertures 17 for forming the base contacts, i.e. comprising the two types of regions to be self-aligned, etching is carried out by conventional photo-lithography over about half of the thickness of layer 14, at the level of apertures 16 and 17. Then (FIG. 4) by using a mask 18 for selection of the base contacts, whose apertures 19 are greater than their counterparts 17 in the composite mask 15, by a distance equal to the maximum possible positioning error attributable to the machines and/or to the operator, the regions of the base contacts are opened, which gives the structure shown in FIG. 5, in which diffusion is then carried out, at the level of the apertures, in regions 20, after removal of mask 18. The photo-etching of the emitters is carried out according to the same principle by means of a mask 21 for selecting emitters 4 (FIG. 6), whose apertures are greater than the corresponding ones in the composite mask, by a distance again equal to the maximum possible positioning error, then diffusion of the emitter(s) 22 is effected, after removal of mask 21.
According to another known self-aligning process described in U.S. Pat. No. 3,951,693, it is possible to reduce the distance between the regions to be doped to a few tenths of a micron. The sequence of operations, illustrated in FIGS. 7 to 11 accompanying the present application, is the following. The formation of the epitaxial layer 23 on a substrate 24, the formation of the masking layer 25, the photo-etching and the diffusion of base 26 are effected conventionally as already indicated in the preceding example. Then a layer of silicon nitride 27 and a silica layer 28 are deposited which are then photo-etched with the same mask, not shown, comprising the patterns of the emitters, and the emitter(s) 29 is implanted after removal of the mask in accordance with FIG. 8. The nitride 27 protected by the silica layer 28 situated above is etched laterally as indicated in FIG. 9 (arrows 30). Then the silica layer 28 is eliminated and another silica layer 31 (FIG. 10) is formed, by an appropriate heat treatment, on the silicon zones not covered by the nitride layer 27. After elimination of this latter, the heavily doped regions 32 of type p.sup.+ (FIG. 11) are formed by doping, which are thus self-positioned with respect to emitters 29, the distance or the spacing shown at d in the figure designating the spacing apart of the differently doped regions 29 and 32 and being of the order of a few tenths of a micron with this technique.
But, the quality of the self-aligning depends on the lateral etching of the nitride. Now, the reproducibility of this latter is difficult to master for it is intimately connected with the silica layer 28, which is itself conditioned by numerous parameters which may vary, such for example as cleaning before depositing the silica and nitride layers, the conditions for depositing, the quality of the interfaces, etc.
With the first process which may be implemented in a reproducible way, the distance between the regions to be self-aligned can only be reduced to about 2 microns, whereas the second process enables this distance to be reduced to a few tenths of a micron, but in a way which is not reproducible.
The present invention has then as its aim to provide a process for self-aligning differently doped regions, which allows self-aligning of said regions to be obtained within a few tenths of a micron.